At the heart of this blame game is the difficult question of who gets defended and who does not.

The blunt truth is the budget has never been big enough to help everyone, so awkward decisions have to be made. The priority is to save people and property.

Calculations are done to assess the value of any flood defence scheme. First, the cost must be outweighed by the benefit - the Treasury insists every pound spent must yield at least eight in economic gain. Second, you add up households at risk and third, you count up deprived households. In all these cases, urban areas are likely to score highest. Farmland is lower in the pecking order.

There are bound to be winners and losers. If the future becomes stormier, and more people are at risk, disputes will become more contentious. And this raises the hardest question of all: how much should we all pay to keep vulnerable areas dry?

Thames Valley Police have declared a "major incident" in east Berkshire.

A major incident has also been declared in Surrey by the county's police force.

Howard Davidson, from the Environment Agency, said he expected conditions in the county to deteriorate as more rain fell over the coming days.

"We have issued flood warnings from Datchet down to Shepperton, and we urge people to take heed of the flood warnings. We are anticipating another three or four inches on the Thames over the next 24 hours."

The Environment Agency said it had never issued as many severe flood warnings and that many areas had seen more than double their average rainfall.

A two-hour meeting of the emergency Cobra committee has taken place.

The prime minister took part by phone from south-west England, where he will spend the night.

Speaking afterwards, Communities Secretary Eric Pickles said sites likely to have problems in coming days were being identified and prioritised.

"Everything possible" was being done to protect homes and communities, and special attention was being paid to water and electrical plants, he added.

The control of sandbags and ordering of temporary flood defences would be centralised by government, he said, and "full military support" remained on standby across the south.

Railway lines have also been badly affected with passengers facing severe delays. The latest developments include:

Trains between Staines and Windsor and Eton have been cancelled until at least Thursday

First Great Western says mainline services from London Paddington had resumed but a reduced timetable was running between Paddington and Reading

The main route into and out of Devon and Cornwall, via Bridgwater, has been cut off by floods and storm damage

First Great Western has lifted ticket restrictions on all journeys

Among other developments:

The Environment Agency has issued 14 severe warnings - meaning "danger to life" - along the River Thames, in areas including Staines, Chertsey and Datchet

Two severe warnings are in place for the South West of England in Salt Moor and East Lyng, in Somerset

Mr Pickles, who is standing in for Environment Secretary Owen Paterson, told MPs that there was a "high risk that the Thames, the Severn and the Wye will flood in the middle of the week".

But he said extra efforts were being made to deal with the problem.

He also denied making "even the slightest criticism of the work of the Environment Agency", amid a row over his suggestion last week that ministers had been given bad advice by the agency over river dredging.

Speaking from Portland, off the Dorset coast, Mr Cameron said: "I am only interested in one thing and that is making sure that everything government can do is being done and will go on being done to help people through this difficult time."

Chancellor George Osborne told ITV's The Agenda: "We're investing in the frontline services now to get these rail lines repaired, to protect homes along the Thames at the moment, the homes in Somerset, bring in the Army and all that kind of thing.

"It does lead to a real debate about how we look after our countryside."

The prime minister spoke to members of the 39 Engineer Regiment working on Chesil Beach in Dorset

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